I. Field of the Invention
This invention relates to linear actuators and to the cooling of said actuators by forced air flow. More specifically the invention relates to a linear actuator which moves a print head in a direction parallel to a print line.
II. Prior Art
The use of linear actuators for positioning print heads is well known in the prior art. A typical linear actuator consists of a magnetic structure, an air gap, and a coil assembly positioned within the air gap. The print assembly is usually connected to the coil assembly and motion is achieved by the magnetic force acting on the coil within the air gap.
Generally, linear actuators may be classified into two groups; based on the manner in which the output is taken from said actuators. In one type of linear actuator the output is taken from a shaft which runs parallel with the horizontal or longest dimension of said actuator. In another type of linear actuator the output is taken from a carriage positioned on the top or open surface of the actuator.
The present invention is more related to the type of actuators where the output is taken from the top. Although the prior art actuators of this type function adequately for their intended purpose, they are plagued by several problems which the present invention will alleviate.
One of the problems which plague the prior art linear actuators is the inefficiency of the linear actuators due to the positioning of the coil in a relatively weak magnetic field. Generally, due to physical arrangement of the prior art linear actuators, only one piece of permanent magnet together with a magnetic loop or return path defines the magnetic circuit. As a result, the number of magnetic lines which are cut by the positioning coil is rather limited and, therefore, the force exerted on the coil, which in turn positions the print head, is relatively low.
One would imagine that the force of these prior art linear actuators would improve by increasing the current density to the positioning coil. However, due to the construction of the prior art coils, the optimum amount of current which can be supplied to the coils is significantly low due to heat generation. Generally, prior art coils are self-supporting. This means the coils are wound from wire strands without any supporting structure and are held together by adhesive substance. With this construction, if current density to the coil exceeds a predetermined value, then the adhesive substance will soften from heat generated by the high current density. Whenever weakening occurs the coil structure, which positions the print assembly, weakens and wobbles as it moves to and fro in positioning the print assembly. This defect further reduces the overall performance of the prior art linear actuators.
Another problem with the prior art devices is that they are relatively expensive. The expense partially stems from the fact that the permanent magnet and other magnetic materials are circular shape. The manufacturing cost of irregular shaped material is significantly more expensive than planar shaped material. As such, the increased machining costs help to increase the overall cost of the prior art linear actuators. By using irregular shaped magnets and magnetic material to form the magnetic circuit of the prior art actuators, it is very difficult to define or set the air gap within which the positioning coil must travel. Due to the importance of the air gap setting to the overall performance of the actuator, the pieces which form the magnetic circuit must be machined with high precision tolerances. This also tends to increase the cost of the prior art linear actuator.
There has been an attempt made to correct some of the aforementioned prior art problems by fabricating a linear actuator using rectangular magnetic structure. Although this structure shows improvement over the prior art devices which are manufactured from irregular-shaped magnetic structure, it also has several problems. Its most outstanding problem is its inefficiency in performance. This stems from the fact that only one surface of the magnet creates flux line for linking the positioning coil. Another problem is that the coil form is rather complicated.
Still another problem which faces the prior art actuators is cooling. Most prior art actuators are cooled by forced air. However, the air is generally forced in a direction parallel to the direction of motion of the coil. However, the air resistance which the coil experienced in its to and fro motion tends to reduce its speed in one direction and increase its speed in the other direction. Also, the prior art coils are not uniformly cooled, which also tends to reduce the efficiency of the linear actuator.
A more detailed discussion of the aforementioned linear actuators is disclosed and discussed in U.S. Pat. No. 3,696,204 and 3,618,514.